Titanium oxide is a metal oxide semiconductor which, upon illumination with photon energy higher than its band gap, produces electrons and holes in the conduction band and valence band, respectively. These photo-generated electrons and holes can combine with surface adsorbed species (e.g., water and oxygen) to form highly reactive radical species such as hydroxyl radical (•OH) and superoxide anion (O2•−). These reactive species have strong oxidizing power and can oxidize most organic compounds. As a result, the presence of these reactive species provides photocatalytic activities to the surface of the titanium oxide particles. Reactive species are typically produced only when titanium oxide is illuminated with UV light (e.g., having a wavelength less than 380 nm).